"Image Enhancement by Histogram Transformation", has been described in a paper bearing that title by Robert Hummel, Computer Graphics and Image Processing, 6, 184-195 (1977):
"The visual contrast of a digitized image can be improved by adjusting the Gray scale so that the histogram of the resulting image is flat. This increase in visual contrast is an artifical enhancement due to a more judicious choice of the relative intensities represented by the quantization levels of the original image. Although the process provides no enhancement in an information-theoretic sense, applying the appropriate transformation can result in a remarkable increase in visual clarity."
Hummel goes on to describe two general categories of transformation systems. The first is defined by Gray-level bins which can only be merged together but wherein no bin can be broken up. If the original histogram has very large peaks the transformed histogram will be only approximately flat. While contrast is improved in a system of this sort, Hummel teaches an improved system wherein very small bins are generated with exactly the right number of points, that is, some of the bins in the original image are broken up and then width is no longer an integral multiple of original bin widths. Hummel also discusses the relative pros and cons of neighborhood enhancement versus global enhancement and suggests the combining of both in some applications.
In a paper entitled "On-Line Image Enhancement In The Time Domain With A Microprocessor" by Z. Orbach et al, IEEE Transactions, April, 1979, pp. 226-231, a system is described which provides histogram gathering and image transformation as semi-independent subsystems controlled by a microprocessor leaving the computationally intensive task of determining the transformation to the microprocessor thereby allowing performance of video enhancement dynamically by means of simultaneous histogram gathering and image transformation. An algorithm is described for the computation required and is applicable to both histogram equalization and histogram hyperbolization. The system requires digitization of the video input by means of an analog-to-digital converter and a reverse transformation from digital to analog by means of a D/A converter at the video output. The digital means provided between the input and output converters is limited by reason of the discrete digital increments produced by the input analog-to-digital converter. The digital system presented is relatively complex.
The A/D converter typically used in prior art employs a series resistor divider for establishing reference voltages for each of the comparators in the A/D converter. See FIG. 1. If the required histogram transformation could be predetermined, the values of R1, R2, . . . and RM might be set to provide the required histogram required for a given degree of video enhancement. However, the desired histogram transformation is a function of the video grey scale distribution in a particular video signal.